Photolithography is used in the production of semiconductor devices to add layers of polymeric material to a silicon wafer and to produce circuit paths within these polymers. Many volatile and non-volatile chemicals are utilized in this process, including solvents, polymer building blocks and other reactive substances. Production of low defect devices at high yield requires extensive quality assurance and quality control activities.
Volatile organic compound (VOC) emission to the atmosphere is a major concern of semiconductor manufacturing industries, research laboratories, the public, and regulatory agencies. Historically, focus has been on cleaning waste VOCs from the manufacturing plant's "air" emissions through the use of scrubbers and filters. Some industries are now seeking ways to reduce emissions by reducing VOCs at the point of use (or generation) to decrease the costs associated with removing VOCs from the air. For successful point of use reduction, VOC measurement methods must be developed for on-line process monitoring. These methods must meet several performance specifications such as rapid response time, continuous detection, lower limit of detection, and speciation of the VOCs detected. Specie-specific information is needed since the chemicals used have different chemical properties as well as different levels at which they become a regulatory concern.
A variety of methods and instrumentation can be used to monitor airborne VOCs depending on the application and the equipment available. Common methods may utilize gas chromatography (GC), mass spectrometry (MS), fourier transform infrared spectrometry (FTIR), chemical specific sensors, or hyphenated techniques including gas chromatography/mass spectrometry (GC/MS). "Protocol for Equipment Leak Emission Estimates," U.S. Environ. Prot. Agency, Off. Air Qual. Plann. Stand., Tech. Rep.! EPA (1993), EPA-453/R-93-026, 257 pp., provides an overview of many methods for monitoring airborne VOCs and field portable GCs.
A system for gaseous VOC monitoring of a lithography process must meet several analytical and physical criteria in order to accurately characterize the emissions. The VOCs in the vapors in the ventilation system are the compounds measured by this invention. Therefore, the analytical requirements of the system, based on the lithography process knowledge and limited flame ionization detector (FID) data, include the ability to 1) detect the particular airborne VOCs used in lithography, 2) attain detection limits for these VOCs below 10 ppm by volume, 3) obtain concentration information for each analyte in the gas stream and 4) acquire at least 1 data scan per second.
Several analytical techniques were examined to assess their ability to meet the requirements described above. Meeting the analytical requirements was the highest priority of the system requirements. The techniques evaluated were gas chromatography (GC), mass spectrometry (MS), GC/MS, .mu.GC, fourier transform infrared spectrometry (FTIR), and FID. Although each technique is capable of detecting lithography VOCs, only MS and FID met or exceeded both the detection limits and data acquisition rate requirements. However, FID could only meet the data acquisition rate requirement when used without chromatographic separation, which does not allow for quantitation of individual analytes. Only mass spectrometry met all the analytical and physical requirements.
Many mass spectrometers can be operated either in electron ionization (El) or chemical ionization (Cl) mode. In El, electrons generated by a hot filament ionize and fragment the analyte molecules. The ionized molecules or fragments are then mass analyzed. Typically, electron ionization is a very energetic process, which causes a high degree of fragmentation of the analyte molecules and leaves few, if any, molecular ions for detection. Identification and quantitation is performed using one or more of the fragment ions.
Chemical ionization differs from electron ionization in that reagent molecules (not electrons) ionize the analyte molecule. For example, for methane Cl, the ionizing reagent molecule is CH.sub.5.sup.+. Methane gas is ionized by electrons and interacts with neutral methane molecules to form a number of products, one of which is CH.sub.5.sup.+. A proton is transferred from CH.sub.5.sup.+ to the sample molecule to form an M+H!.sup.+ ion where M is the molecular weight of the sample molecule. Therefore, the parent ion in chemical ionization appears in the mass spectrum at a mass which is 1 greater than the molecular weight of the neutral analyte molecule.
Chemical ionization is much softer (less energetic) than electron ionization; this affords significant advantages for airborne VOC measurement when mixtures are present. The Cl analyte molecular ion signal is more intense and fewer fragment ions are produced than with El, which minimizes the mass spectral interferences and causes Cl to be more sensitive than El for many compounds.
Because a mass spectrometer counts the number of ions over a period of time, quantitative measurements require that the MS be calibrated against a known source operating at the same pressure as the source to be tested. It is difficult to use mass spectrometry for on-line VOC measurements because various sources of VOCs operate at different pressures, and these pressures change during operation due to ventilation or barometric changes, which means the MS must be recalibrated with each pressure change.